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<title>Using RTL SSA (GNU Compiler Collection (GCC) Internals)</title>

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<a name="Using-RTL-SSA"></a>
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<p>
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<hr>
<a name="Using-RTL-SSA-in-a-pass"></a>
<h4 class="subsection">14.21.1 Using RTL SSA in a pass</h4>

<p>A pass that wants to use the RTL SSA form should start with the following:
</p>
<div class="smallexample">
<pre class="smallexample">#define INCLUDE_ALGORITHM
#define INCLUDE_FUNCTIONAL
#include &quot;config.h&quot;
#include &quot;system.h&quot;
#include &quot;coretypes.h&quot;
#include &quot;backend.h&quot;
#include &quot;rtl.h&quot;
#include &quot;df.h&quot;
#include &quot;rtl-ssa.h&quot;
</pre></div>

<p>All the RTL SSA code is contained in the <code>rtl_ssa</code> namespace,
so most passes will then want to do:
</p>
<div class="smallexample">
<pre class="smallexample">using namespace rtl_ssa;
</pre></div>

<p>However, this is purely a matter of taste, and the examples in the rest of
this section do not require it.
</p>
<p>The RTL SSA represention is an optional on-the-side feature that applies
on top of the normal RTL instructions.  It is currently local to individual
RTL passes and is not maintained across passes.
</p>
<p>However, in order to allow the RTL SSA information to be preserved across
passes in future, &lsquo;<samp>crtl-&gt;ssa</samp>&rsquo; points to the current function&rsquo;s
SSA form (if any).  Passes that want to use the RTL SSA form should
first do:
</p>
<div class="smallexample">
<pre class="smallexample">crtl-&gt;ssa = new rtl_ssa::function_info (<var>fn</var>);
</pre></div>

<p>where <var>fn</var> is the function that the pass is processing.
(Passes that are <code>using namespace rtl_ssa</code> do not need
the &lsquo;<samp>rtl_ssa::</samp>&rsquo;.)
</p>
<p>Once the pass has finished with the SSA form, it should do the following:
</p>
<div class="smallexample">
<pre class="smallexample">free_dominance_info (CDI_DOMINATORS);
if (crtl-&gt;ssa-&gt;perform_pending_updates ())
  cleanup_cfg (0);

delete crtl-&gt;ssa;
crtl-&gt;ssa = nullptr;
</pre></div>

<p>The <code>free_dominance_info</code> call is necessary because
dominance information is not currently maintained between RTL passes.
The next two lines commit any changes to the RTL instructions that
were queued for later; see the comment above the declaration of
<code>perform_pending_updates</code> for details.  The final two lines
discard the RTL SSA form and free the associated memory.
</p>
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